Review Question - QID 213680

A 34-year-old woman with a past medical history of IV drug abuse presents to the emergency department with a fever and altered mental status. The patient is homeless and has presented many times in the past for intoxication. Her temperature is 103°F (39.4°C), blood pressure is 94/54 mmHg, pulse is 129/min, respirations are 12/min, and oxygen saturation is 98% on room air. Physical exam is notable for multiple abscesses in the antecubital fossa, tachycardia, and a murmur over the right sternal border. The patient becomes progressively less responsive while in the emergency department, and her blood pressure declines despite IV fluids and a norepinephrine drip. An initial chest radiograph and urinalysis are within normal limits. A central venous line is attempted under ultrasound guidance with the finding in Figure A. The patient has basic labs and blood cultures drawn and is started on broad spectrum antibiotics and vasopressors. She is sent to the ICU for further care. Prior to being sent to the ICU, which of the following diagnostic tests should be performed for definitive confirmation of central line placement?